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We have discussed in previous post about causes of hydraulic system overheating , cost of hydraulic oil leaks and "Reasons of hydraulic hose failure".

Today we will discuss difference between closed loop and open loop hydraulic system/unit

Closed Loop Hydraulic system

In case of closed loop hydraulic system, Hydraulic fluid will flow from pump to actuators i.e. hydraulic motor and actuators to pump continuously. Hydraulic fluid will enter at inlet of pump after passing through the actuators. One hydraulic pump might be used for driving the multiple hydraulic motor in case of closed hydraulic system.

Hydraulic pump will deliver the fluid to actuators and will receive the same quantity of fluid from its inlet port from the actuators for smooth operation of the system. However, one feed pump is always provided with closed loop hydraulic system in order to make-up the fluid in closed loop circuit. Feed pump will be a fixed displacement having capacity approximate 15 % of main pump.

In case of closed loop hydraulic system, main hydraulic pump can deliver the high pressure fluid from it’s both ports and therefore main pump will control the direction of rotation of hydraulic motor i.e. main pump can control the direction of rotation of hydraulic motor. 

Main pump can also control the pressure and flow of fluid in closed loop hydraulic system and that’s why controlling elements such as direction control, flows and pressure regulating elements will not be required for closed loop hydraulic system. 

Closed loop hydraulic system is suitable for precise working but heat generation in this case will be higher as compared to open loop system.

You might be thinking, why I am using hydraulic motor only as actuator for closed loop hydraulic system?
This is only because in case of hydraulic cylinder there will be difference in fluid volume displacement during extension and retraction of cylinder and hence closed loop hydraulic system will not be a feasible for hydraulic cylinders applications.

Open Loop Hydraulic system

In case of open loop hydraulic system, hydraulic pump will suck fluid from reservoir and will deliver the high pressure fluid to the actuators i.e. hydraulic cylinders via passing through pressure relief valve , pressure regulating and flow control valve. 

After passing from actuators, fluid will go to heat exchanger and then reservoir and again pump will take fresh fluid from reservoir via its inlet port and this process will be continuous during operation of hydraulic actuators. 

In case of open loop hydraulic system, pump will deliver the high pressure fluid in one direction and hence direction control valves will be used for controlling the direction of movement actuator. Hydraulic pump will have larger diameter port at its low pressure side i.e. inlet side and will have smaller diameter port at its high pressure side i.e. at outlet. 

Heat generation in open loop hydraulic system will be less but as large volume of fluid will be required in case of open loop system, hence size and weight of open loop system will be more as compared to close loop system.

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